In settings>general you can set how you want messages opened. To delete multiple messages, just highlight in the message list pane and click delete. From your post, it seems you are clicking to open and then delete.

The only items listed under Settings / General are: Thunderbird Start Page, Default Search Engine, System Integration. None of these offer any options for how it should open messages.

Maybe you should read more carefully. See attached screenshot . If you look carefully, there is a scroll bar on the right.

Yes, but that appears under Settings / Reading & Display, not Settings / General. Even so, this option merely directs how/where the message should be opened.

I don't want to open it at all when I simply click on it; I just want to select it. If I want to open it, I used to double-click; that's the way I like it. How can I restore this functionality?

Actually, 'reading&display' appears under Settings>General, but I won't pursue that debate. If you have the vertical view, clicking once displays the message. Double-clicking opens in a new tab. What view are you using?

Automatically opening messages that are just selected, not double-clicked, is VERY annoying. I get many messages every day. I start out by selecting (not opening) all the day's messages in the Inbox, then unselect the messages that I want to keep. Then I delete the rest. It takes a huge amount of time to then examine and maybe close every opened message tab.

I suggest testing in troubleshoot mode, as that disables addons. I am unable to duplicate the problem of messages being selected opening.